1. Field of the Invention
The present invention relates to a two degree of freedom position control method, two degree of freedom position control device and medium storage device using both feedback and feed forward, and more particularly to a two degree of freedom position control method, two degree of freedom position control device and medium storage device for performing two degree of freedom control using observer control.
2. Description of the Related Art
A device for controlling the position of an object to a target position is widely used. For example, this device is used for seek control for moving a head to a target track, which is one of the positioning controls of a disk device, such as a magnetic disk device and an optical disk device.
A method for this seek control is using two degree of freedom control. In the two degree of freedom control, a target position is provided via a filter while forming a feedback loop. In other words, the transfer function from a target position to an observation position is set to be a form of a low pass filter. By this, overshoot (overrun) can be suppressed effectively.
In the observer control system which is generally used for disk devices as well, two degree of freedom control system can be constructed. In this case, the control system has a form of a second degree low pass filter (hereafter called LPF), which has a same pole as the feedback pole. FIG. 12 is a block diagram of a two degree of freedom control of a prior art.
As FIG. 12 shows, a target trajectory generation section 100 generates a target trajectory r(n) from a target position r. The target trajectory r(n) indicates a target position which moves between samples determined for each sample. A position error computing section 102, on the other hand, computes an error y[n] between the target position ‘r’ and a current position ‘y’ observed from a plant 106. A controller 104 receives the target trajectory r(n) and the position error y[n], performs the computation of the two degree of freedom observer, calculates a drive command value of the plant 106, and drives the plant 106.
As an observer used for the controller 104, a two degree of freedom control observer, shown in FIG. 13 and FIG. 14, has been proposed (e.g. “Digital Control of Dynamic Systems”, (by Gene F. Franklin and two others, published by Addison-Wesley, 1998)). As FIG. 13 shows, an observer (estimator) 104-1 is used for the controller 104. This estimator 104-1 computes the output u[n] from the current position ‘y’ of the plant 106 and the output to the plant 106, and multiplies its output by an open loop gain K in a gain multiplication block 104-2, and the result is fed back.
In a multiplication block 104-4, the target trajectory r[n] is multiplied by a coefficient N, the result is output to an addition block 104-3 and added to the output of the gain multiplication block 104-2 so as to calculate the output u[n] to the plant 106.
FIG. 14 shows a simplified observer of this two degree of freedom control which is expressed by the following Expressions (1), (2) and (3).Xh(n)=Xb(n)+L(y(n)−C·Xb(n))  (1)u(n)=−F·Xh(n)  (2)Xb(n+1)=A·Xh(n)+B·u(n)  (3)
In other words, the difference between the observation position y(n) in the current sample ‘n’ and the estimated position C·Xb(n) of the current sample estimated with the previous sample is computed in a computation block 202, and an estimated position error er[n] is generated. In a multiplication block 204, this estimated position error er[n] is multiplied by an estimated gain L so as to generate a correction value.
In an addition block 206, this correction value and Xb[n] such as an estimated position and an estimated velocity, are added. By this, Xh(n), such as an estimated position and estimated velocity in the current sample, is generated using Expression (1). In the case of ordinary state feedback, the estimated position of the estimated state Xh(n) is multiplied by a gain, and estimated velocity is multiplied by the gain, and the sum thereof is determined to generate state feedback current.
In the above mentioned two degree of freedom control, the value when the estimated velocity is multiplied by the gain is still used, but the difference value between the estimated position Xh(n) and the target position trajectory r(n) is computed in the addition block 210, the result is multiplied by the feedback gain F in a multiplication block 212, and this result is used for the state feedback. In other words, Expression (2) is computed.
On the other hand, the estimated state Xb(n+1) of the next sample (n+1) is computed from the estimated state Xh(n) in the current sample and the output value u(n) using Expression (3) in multiplication blocks 214 and 216 and an addition block 218.
Here A, B, C, C^T, L and F are matrixes for position x, velocity v, bias value b and disturbance values d1 and d2. A, B and L are state estimate gains, F is a feedback gain, and C and C^T (transposed matrix) are given by the following Expression (4) and (5).
                    C        =                  (                      1            ⁢                                                  ⁢            0            ⁢                                                  ⁢            0            ⁢                                                  ⁢            0            ⁢                                                  ⁢            0                    )                                    (        4        )                                          C          T                =                  (                                                    1                                                                    0                                                                    0                                                                    0                                              )                                    (        5        )            
The current observer indicated by Expression (1), Expression (2) and Expression (3) and FIG. 14 can implement two degree of freedom control only by multiplying the target trajectory r(n) by C^T of Expression (5), and adding the result to the normal current observer.
The above mentioned two degree of freedom control of prior art suggests a single rate control, which means that the drive current is changed once in one sample, as shown in FIG. 15. In other words, as FIG. 15 shows, the position in a current sample ‘n’ is observed, two degree of freedom control computation is performed once, and the drive current u(n) is output.
However in digital control, computation processing is performed by a microcontroller, so an output delay time is generated. Along with the recent demand for high-speed moving and high precision positioning, a control delay is becoming conspicuous in a conventional method of estimating the state once in one sample to change the drive current.
For example, in the case of the magnetic disk device, density is set to high, several tens of thousand tracks are on one disk face, and a high moving speed is demanded, so a state change becomes major during calculation even if state is estimated once and the device current is changed once in one state estimation, and an output delay makes highly accurate position control difficult, and an overrun easily occurs.